In order to implement a 2-wire video intercom system, the direct current power, the video carrier signal, the audio signal and the command data signal have to be transmitted on a common 2-wire bus simultaneously. Therefore, the power supply circuits of the system power supply and devices thereof have to comprise an inductance component connected to the common bus in series, which allows the direct current passing whilst also suppresses the alternating current signal. However, it is common sense to the skilled person in art that the audio signal frequency is low to 300 Hz if a coil inductor is used as the inductance component. In order to achieve the sufficient impedance and power supply capacity as well, the size of such inductor shall become very large. FIG. 1 illustrates a circuit diagram of a power supply circuit for a 2-wire bus intercom system according to the prior art. As shown in FIG. 1, an electronic inductance circuit can be usually used to replace the coil inductor to reduce its size. In detail, FIGS. 2a-2d individually illustrate a circuit diagram of 4 conventional power supply circuits for a 2-wire bus intercom system according to the prior art. However, there is only one system power supply for a 2-wire intercom system in a building; hence, even for a large 2-wire intercom system with large numbers of device loads, it only allows a very small number of devices at operating status whilst the absolute majority of the device loads have to be at standby status.
As above described, the conventional power supply circuits for a 2-wire intercom system are consisted of an electronic inductance circuit and a voltage regulator circuit, which offer power to both standby circuits and operation circuits. While the alternating current impedance of an electronic inductance circuit is limited, especially a large current power supply with an electronic inductance circuit for an operation circuit. However, all the device loads of a 2-wire intercom system are connected to the common bus in series, which means the alternating current impedance of n device loads is 1/n of one device load. When the quantity of device loads is large, the alternating current impedance will become insufficient, the size of 2-wire intercom system will be limited and the audio quality of such 2-wire intercom system will become much worse.
Consequently, existing solutions including the prior art mentioned above can't achieve a higher alternating current impedance at the standby status when the 2-wire bus intercom system is connected with large amount of device loads. Due to the above mentioned problems, the present invention is to propose a standby power supply circuit for a 2-wire bus intercom system and an apparatus thereof.